Among manganese-zinc system ferrites, high magnetic permeability ferrites comprising 50 to 56 mol % of Fe.sub.2 O.sub.3, 22 to 39 mol % of MnO, and 8 to 25 mol % of ZnO, for example, have been widely used as magnetic cores for wide frequency band transformers and magnetic cores for line filters and noise filters of various communication equipment or the like. These high magnetic permeability ferrites are now required to have high magnetic permeability particularly in the high-frequency range of about 100 kHz to about 500 kHz.
Manganese-zinc system polycrystalline ferrites have the general tendency that initial magnetic permeability .mu.i increases with a crystal grain size. One exemplary substance known to promote grain growth is Bi.sub.2 O.sub.3, which is practically used in Japanese Patent Publication (JP-B) No. 29439/1977 to establish a mean grain size of 50 .mu.m or greater for improving .mu.i. However, a greater grain size concurrently entails increased losses, especially at high frequencies, due to which only lower frequencies are available.
Then JP-B No. 49079/1976 attempted to improve high-frequency response by adding Bi.sub.2 O.sub.3 and CaO in admixture. This improvement, however, is effective only at frequencies of up to about 100 kHz, and this material is not practically acceptable as the high magnetic permeability material for line filters or the like which is required to have high .mu.i at higher frequencies, for example, over the entire range of from 10 to 500 kHz.